A digitizer cursor is part of a digitizing device that, in conjunction with a digitizing tablet, is used to generate digital signals representative of the location of the cursor on the tablet. Digitizing devices are often used with data processing equipment to allow the operator to input information that can best be expressed in multi-dimensional analog terms. For instance, a digitizing device can be used in conjunction with computer aided design equipment to enable the user to input information regarding the location of features of the item being designed. Digitizing devices can also be used as part of a communications system to enable the users to transmit and receive digital representations of analog information.
A typical digitizing device includes a digitizing tablet, a digitizing cursor, and a digitizing circuit. The digitizing tablet is provided with two sets of parallel grid conductors that are electrically connected to the digitizing circuit. The individual grid conductors are spaced apart from each other and the sets are arranged perpendicular to each other so as to form an x-y grid of rectangular sections separated by the conductors.
The digitizing cursor includes a relatively flat lens assembly typically formed from two sections of planar, spaced apart transparent material. Cross hairs, arranged to form a cross hair center point, are inscribed on one of the sections of transparent material, usually the section placed closest to the digitizing tablet. A digitizing cursor coil, typically wire wound on a bobbin, is housed inside the viewing assembly between the two sections of transparent material. The digitizing coil is connected to the digitizing circuit and is energized thereby. The digitizing coil is positioned inside the viewing assembly so that when a current is supplied to it, the center point of the magnetic field formed around the coil is aligned with the center point of the cross hairs.
A digitizer typically is provided with an overlay above the digitizing tablet that is representative of the area containing the coordinates that the user desires to digitize. The user identifies, or digitizes, specific coordinates on the overlay by first positioning the cursor so the lens cross hairs are aligned over the desired location. The cursor coil is then energized, a current is applied to it, so the resulting cursor magnetic field is generated. Sensors in the digitizing circuit associated with the individual grid conductors in the digitizing tablet provide an indication of the center point of the cursor coil magnetic field on the digitizing tablet. The signals produced by the sensors are digitized by additional components within the digitizing circuit to provide the digital signals representative of the cross hair center point's location on the digitizing tablet.
Advances in electronics technology have made it possible to interpolate the position of the lens cross hairs/cursor coil between parallel grid conductors. This has made it possible to provide digitizing devices that generate an output of the location of the cursor on the digitizing tablet with a very high degree of accuracy.
An important consideration in the design of digitizing devices is the elimination of edge effects produced by the cursor coil when it is energized. Edge effects are caused by fringe magnetic fields that invariably form within the area spanning a certain radial distance away from the cursor coil.
Edge effects are undesirable because the fringe magnetic fields cause some of the sensors associated with the grid conductors to generate extraneous signals that interfere with the processing of the signals that represent the true location of the cursor. Consequently, an important consideration in the manufacture of digitizers is providing a means to eliminate the unwanted fringe magnetic fields.
Currently, edge effects around a cursor coil are reduced by providing a metal shield around the coil. This metal shield typically takes the form of a metal ring which surrounds the cursor coil. Additional shielding is provided by mounting the lens assembly, including the cursor coil, on a metal base, typically made of aluminum. The metal ring and metal base provide sufficient shielding so that the edge effects and attendant problems are eliminated.
However, there are several disadvantages associated with using metal shielding to eliminate edge effects. Assembling the cursor so the bobbin and wire forming the cursor coil are secured in the metal ring is a complicated and costly task. Securing the cursor coil is difficult in part because the coil must be positioned so that, when it is energized, the center point of its magnetic field is aligned with the lens cross hair center point. Providing an assembly which permits the bobbin forming the cursor coil to be so aligned and positioned is expensive.
Furthermore, the metal shield in the cursor adds to its bulk and weight making the cursor more cumbersome to manipulate during use of the digitizer.